A number of thermostats have been designed to give a snap action rather than a creep action by the temperature change of a thermostat element, and many of these snap acting thermostats have utilized a thermostatic disc. A few have been of a first type to utilize the force of the disc to close the electrical contacts in the thermostat, and these include U.S. Pat. No. 2,954,447, issued Sept. 27, 1962 to J. D. Bolesky et al; U.S. Pat. No. 3,197,594, issued July 27, 1965 to Donald J. Schmitt; U.S. Pat. No. 3,248,501, issued Apr. 26, 1966 to C. J. Hire; and U.S. Pat. No. 3,451,028, issued June 17, 1969 to Donald J. Schmitt. In the second-mentioned of the foregoing patents, the current flow was through the metallic disc. In the third of these patents, the design was a double-throw contact switch, and hence inherently in one of the two throw positions of the disc, force was used to close the contacts. A more usual design was found in a second type wherein the force of the thermostatic disc was used to open the contacts rather than to close them, and this design is shown, for example, in U.S. Pat. No. 4,367,452, issued Jan. 4, 1983 to Richard H. Carlson.
In a few thermostat designs, an attempt has been made to achieve a longer resilient contact strip than the space permitted within the thermostat housing. Examples of this third type of thermostat are found in U.S. Pat. No. RE 26,406, issued June 11, 1968 to John D. Bolesky. in U.S. Pat. No. 3,621,434, issued Nov. 16, 1971 to Anton J. Gerich, and in U.S. Pat. No. 4,367,452, supra. In a fourth type of thermostat, the resilient contact strip has a hinge portion closely adjacent the mounting end of the contact strip, is actuated by application of force at a point outboard of the hinge, and at an area of the contact strip which has no substantial flexing, established, e.g., by stiffening ribs. This fourth type of thermostat is illustrated in U.S. Pat. No. 3,537,052, issued Oct. 27, 1970 to Harold F. Snider; U.S. Pat. No. 3,601,741, issued Aug. 24, 1971 to Ronald L. Holden; and U.S. Pat. No. RE 28,019, issued May 28, 1974 to Donald J. Schmitt. Many other thermostats are of this type, but because the use of stiffening ribs was so common, many of the thermostat patent drawings did not bother to disclose them.
A fifth type of prior art thermostat is one wherein an edge of the thermostatic disc is that which is used to actuate the contact strip. This type of thermostat is illustrated in U.S. Pat. No. 3,322,920, issued May 30, 1967 to Rexford M. Morris; U.S. Pat. No. 3,541,488, issued Nov. 17, 1970 to Clifford S. Odson; U.S. Pat. No. 3,660,793, issued May 2, 1972 to Edward G. Them et al; and U.S. Pat. No. 3,852,698, issued Dec. 3, 1974 to Donald J. Schmitt.
These various prior art designs have not solved the problem of how to establish a miniature thermostat which is a snap acting thermostat, wherein the maximum dimension thereof is less than 3/8 inch. Such a thermostat may utilize a thermostatic disc for maximum snap acting force developed in such a small area, yet the prior art designs have been ones wherein it was exceedingly difficult to achieve sufficient contact separation in the open circuit condition to provide a satisfactory thermostat which had sufficient voltage and current ratings and sufficient cycles of operation to be a marketable thermostat.